This invention relates to hair treatment compositions. In particular the invention relates to hair treatment compositions comprising a phase formed from a particular type of gel.
A suspending agent is commonly employed in hair treatment compositions to improve stability against phase separation and settling of suspended materials.
Examples of commonly used suspending agents include crystalline suspending agents (such as ethylene glycol distearate) and inorganic structurants (such as swelling clays). Although these materials are effective for suspending particulate matter, they can adversely affect lathering performance and impart an undesirable cloudy appearance to the composition. Furthermore, during use of the composition they tend to get co-deposited along with the ingredients it is desired to deposit, which can lead to dulling of the hair through excessive build up and reduced performance.
The prior art also proposes the use for suspending purposes of hydrophilic polymers which disperse in aqueous media. Natural polymers have been used for this purpose, and in particular xanthan gum has been used. Personal washing products, especially shampoos, containing xanthan gum are described for example in U.S. Pat. No. 5,286,405 and GB-A-2188060.
A problem is that the resulting products often have an unacceptable xe2x80x9cstringyxe2x80x9d texture and a slimy feel.
One category of synthetic polymers used for suspending purposes are carboxyvinyl polymers. The carboxyvinyl polymers are colloidally water soluble polymers of acrylic acid cross-linked with polyallylsucrose or polyallylpentaerythritol, obtainable under the CARBOPOL trademark from B F Goodrich. U.S. Pat. No. 5,635,171 describes a transparent or translucid gel based on such polymers, in which the gel is rigidified by the incorporation of a very small quantity of an aqueous solution of galactomannan (carob, guar or tara gum). This rigidification enables the stabilization of suspended phases.
A problem is, however, that carboxyvinyl polymers of the above described type can be difficult to formulate because of, inter alia, their sensitivity to pH and ionic strength and their incompatibility with ethoxylated surfactants.
A number of polymers of biological origin, when in aqueous solution, have the ability to form so-called reversible gels which melt when heated but revert to a gel when cooled down subsequently. One well known example of a polysaccharide which forms reversible gels is agar. An aqueous solution containing a small percentage of agar is a mobile liquid when hot, but when left to cool it forms a gel with sufficient rigidity to maintain its own shape. Other naturally derived polymers which can form reversible gels are carrageenan, furcelleran, gellan and pectin.
The formation of gels by natural polysaccharides arises from interaction between the polymer molecules. Reversible gels generally display a melting temperature or temperature range, referred to as the gel point. This is the temperature at which, on slow heating, the gel is observed to melt as this interaction largely disappears. Thus, above the gel point, the hot solution of polymer is mobile. When it cools below its gel point, the interaction of polymer molecules enables them to form a continuous and branched network which extends throughout the sample. In contrast with the formation of a continuous, branched network, some other materials which thicken water do so through merely local, transient entanglement of molecules. A discussion of polysaccharide gels, including their range of mechanical properties, is found in xe2x80x9cGels and Gellingxe2x80x9d by Allan H Clark which is Chapter 5 in Physical Chemistry of Foods, Schwartzberg and Hartel, editors; published by Marcel Dekker 1992. In some instances there is hysteresis and the melting and setting temperatures are not identical.
The melting temperature of a gel can suitably be measured by placing a steel ball, having a diameter of approximately 1 mm, on the surface of a sample which is fully set, then raising the temperature slowly, e.g., in a programmable water bath. The gel melting point is the temperature at which the ball begins to sink through the sample. Apparatus to facilitate such determinations is available, for example as a Physica AMV200 rolling ball viscometer from Anton Paar KG.
A reversible gel also displays a transition temperature at which, upon slow temperature increase, all ordering, be it of microscopical or macroscopical extent, has disappeared completely. This transition temperature (from order to disorder) can be measured by means of differential scanning calorimetry (DSC). The transitions temperature of a reversible gel, as measured by DSC, usually approximately coincides with gel melting, observable visually.
EP-A-355908 teaches that polysaccharides which are capable of forming a reversible gel can be used to form viscous, yet mobile, fluid compositions by subjecting the composition to shear while gel formation takes place. The resulting compositions can be termed xe2x80x9cshear gelsxe2x80x9d.
We have now found that hair treatment compositions comprising a continuous phase formed from such shear gels display excellent resistance to phase separation and settling of suspended materials. The shear gels are tolerant to the presence of surfactant, and may under some circumstances boost the conditioning performance of the hair treatment composition.
WO98/08601 describes aqueous compositions such as liquid personal cleansers containing large hydrogel particles formed by two different water soluble polymers. The hydrogel particles trap water insoluble benefit agents in a network formed by these two polymers. The system is not a shear gel since it is prepared by first forming elongated polymer gel noodles which after gel formation are subsequently cut/broken into the desired gel particle size. The second polymer (which is typically an acrylic polymer such as CARBOPOL referred to above) is required to modify gel strength in order to help stabilize benefit agent in the polymer hydrogel system.
WO95/12988 refers to suspensions or dispersions of gelled and hydrated biopolymer particles for use in food or personal care products to impart a fatty-like character to the product. This system is not a shear gel since particulation of dry material at a temperature equal to or above T(gel) is followed by hydration of the particles at a temperature lower than T(gel), the term xe2x80x9cT(gel)xe2x80x9d denoting the temperature at which, upon cooling, an aqueous solution of the biopolymer concerned, sets to a gel.
In a first aspect, the present invention provides a hair treatment composition which has a thickened fluid form comprising:
(i) a first (shear gel) phase comprising at least one naturally derived polymer which is capable of forming a reversible gel, which polymer is present in the composition as a shear gel (i.e., a multiplicity of separate gel particles which have been formed by subjecting the polymer to shear while gel formation takes place), and
(ii) a second (suspended) phase suspended therein.
In a second aspect, the invention provides the use of a shear gel as a suspending system in a hair treatment composition.
In the present specification, the expression xe2x80x9cthickened fluidxe2x80x9d is used to denote a composition with viscosity greater than that of water.
In order that the gel particles remain stable in the presence of surfactant (which will normally be present in hair treatment compositions of the invention), it will generally be desirable that the polymer does not require polyvalent cations in order to form the precursor aggregates that are subsequently capable of intermolecular association leading to formation of a gel network. Consequently, it is desirable that the polymer is capable of forming a reversible gel when dissolved at a sufficient concentration in hot distilled or demineralised water and allowed to cool to an ambient temperature of 20xc2x0 C.
Compositions embodying this invention may be made with viscosities in a wide range. At one extreme, the compositions may be freely mobile, self-levelling and pourable, although thicker than water. On the other hand, they may be made as viscous liquids which can be squeezed from a collapsible container, and yet which are too viscous to pour, except very slowly.
They are shear-thinning, which can be a useful property in hair treatment compositions such as shampoos and conditioners, because the user can perceive the product as thick and viscous, and yet find it easy to apply. An advantage of viscous shear gels is that they are good at retaining the shape which has been squeezed out, and so can be dispensed by methods other than simple pouring such as from flexible or deformable squeeze tubes.
If the compositions are heated to a temperature above the melting and transition temperatures, the individual gel particles will melt and will not reform as separate particles on cooling, but this will not be a problem in ordinary use, because reversible gels generally have melting temperatures well above normal room temperatures.
Viscosity of compositions embodying this invention can be measured using the same techniques as are used to measure viscosities of other thickened liquid compositions. One suitable apparatus is the Haake Rotoviscometer, another is the Carri-Med CSL 500 viscometer.
Many compositions of this invention will display a viscosity in a range from 0.1 Pa.s to 1000 Pa.s at a shear rate of 10 secxe2x88x921 measured at 20xc2x0 C.
One route for the preparation of the sheared gel particles required for this invention starts with the provision of an aqueous solution of the polymer, at a temperature above the gel melting temperature (and probably also above its order to disorder transition temperature), then cooling the solution to a temperature below the gel setting temperature, while applying shear to the composition. Generally, the solution will be subjected to shear while cooling from 60xc2x0 C. or above to 25xc2x0 C. or less.
On a small scale, this may be carried out in a beaker with a mechanical stirrer in the beaker, providing vigorous stirring while the contents of the beaker are allowed to cool.
We prefer to carry out the preparation using a scraped surface heat exchanger. This may be equipped to operate under a partial vacuum to reduce the incorporation of air bubbles into the composition as gel formation takes place.
Another possibility for preparing the gel particles is to form a bulk quantity of the gel and then break this up into small particles, for instance by pumping it through a homogeniser.
We have found that for many polymers gel formation is inhibited by the presence of surfactant (which is normally a component of hair treatment compositions), and yet gel particles which have already been formed remain stable if surfactant is added subsequently.
Therefore, generally it will be desirable to form the gel particles by cooling an aqueous solution of the gel-forming polymer in the substantial absence of surfactant, and then add surfactant subsequently. An alternative approach is to incorporate surfactant into the aqueous composition before the step of cooling under shear, but this is not possible for all gel-forming polymers.
Thus, in a further aspect, this invention provides a method of preparing a hair treatment composition as set forth above which comprises forming a hot, mobile aqueous solution of the polymer, cooling the solution through its gel temperature, subjecting it to shear during or after cooling, and incorporating surfactant possibly before but preferably after cooling through the gel temperature.
A laboratory-scale scraped surface heat exchanger which we have used successfully is the ESCO Labor mixer available from ESCO Labor, CH-4125, Reihen, Germany.
Scraped surface heat exchangers and homogenisers are used in the commercial production of margarine and other spreadable foodstuffs and such apparatus may be used to produce compositions of this invention on a larger scale. A discussion of such heat exchangers is given by Harrod in Journal of Food Process Engineering 9 (1986) pages 1-62. Suppliers of such apparatus include Armfield Ltd, Ringwood, Hampshire, England, Contherm Corporation which is a division of the Alfa-Laval Group, USA and APV Projects (Crepaco) Ltd, Crawley, West Sussex, England.
After the formation of gel particles, the addition of surfactant or other ingredients, probably as a liquid concentrate, can be carried out using conventional mixing apparatus, operating at low shear. Possibly a scraped surface heat exchanger used to form the gel particles can also be used for a subsequent mixing operation, especially if run more slowly, so as to give lower shear. A mixing operation should not be allowed to heat the composition sufficiently to cause the melting of the gel particles. If necessary, a composition containing gel particles should be cooled before and/or during any subsequent mixing operation.